Determining the dynamic and/or static light scattering makes it possible to determine the particle size, particle shape, diffusion coefficient, and molecular weight of substances. To make this determination, a light source emitting coherent light and a sensitive single-photon detector are used to measure the total light intensity (static light scattering) and changes in light intensity as a function of diffusion processes (dynamic light scattering) of a sample in a solution. Any change in the scattering angle and thus the wave vector of the scattered light provides additional information in studying either static or dynamic light scattering. The total intensity of scattered light measured for static light scattering increases with the volume of the sample and with the intensity of the incident light. For dynamic light scattering, changes in light intensity of the coherent scattered light emitted by the sample at a given time are compared with the scattered light emitted by the same sample at another point in time. Small sample volumes or, alternatively, when any desired liquid volume is to be used, coherent light must be used to ensure that coherent mixture of the scattered light takes place.
Instruments for determining light scattering must allow measurement of both static and dynamic light scattering. This, however, is only possible if a compromise is found between the use of large volumes for static light scattering and small volumes for dynamic light scattering.
Therefore, performing static and dynamic light scattering tests consecutively, using an optimized detector for each measurement, has been contemplated. This idea, however, has been rejected, since in consecutive measurements the sample being tested is no longer the same from test to test, and neither the optical nor the sample parameters are the same, but have changed during the time between the consecutive measurements. Obvious examples include dynamic changes in the sample due to jelling, polymerization, or crystallization occurring between two measurements, as well as samples with a high degree of heterogeneity. Consequently, comparable results cannot be expected with consecutive measurements in each of these mentioned cases.
The need to perform angle-dependent measurements and thus to modify the wave vector points produces another difficulty. That is, it must be ensured that it is always the same sample volume that is being measured when testing the sample with the most diverse scattering angles. This practice takes into account the theoretical dependence of the resulting effective scattering volume, allowing subsequent corrections, and prevents small inhomogeneities from resulting in fully unexpected results. In order to assure this, a goniometric procedure is normally performed. The sample is adjusted, so that it is located exactly in the center of rotation of a rotary platform with a swivel arm carrying a detector. The laser light is adjusted to the sample in the center of rotation. This ensures that the same scattering volume is measured in the entire scattering angle range as long as the required adjustment is accurate along all axes, i.e., both in the horizontal and vertical directions, at the inclination or angular deviation of the detector and the laser beam.
Although these adjustments must usually be very accurate and require an accuracy better than 10 .mu.m for most parameters, they can be made within these strict parameters if the detector is brought to the zero-degree scattering angle position, where it is hit directly by the laser beam. The procedure for adjusting a commercially available device for measuring light scattering is known.
Nevertheless, for many samples simultaneous measurement of static and dynamic light scattering using consecutive measurements from several scattering angles represents a problem. Satisfactory results are not always obtained in consecutive measurements whether due to sample instability caused by jelling, polymerization, and/or crystallization, or due to other limitations such as the measuring time available for each sample. There are also standard tests where static and dynamic light scattering are measured consecutively from different scattering angles, for example, in measuring particle size, without the test results being necessarily affected. The scattered light may be observed simultaneously by a plurality of detectors from different viewing angles. See, German Patent 38 13 718 A1. This, however, fails to provide a solution in that it is extremely costly to adjust a plurality of detectors so that each detector receives scattered light from the same scattering volume. Moreover, the number of scattering angles available for measuring is limited by the number of the detectors used around the sample.